As will be apparent from the above-listed patents and applications, an exhaust-gas-cleaning installation for the scrubbing of the waste gas of a pressurized blast furnace generally comprises at least the following components:
(a) A wash tower connected to the gas-discharge port of the blast furnace and provided with at least one annular-gap washer whose gap is controllable (controlled-gap washer) and which is responsive to the pressure differential thereacross so that the gap is adjusted in respect to this pressure differential.
(b) An expansion turbine connected to the wash tower for at least partial recovery of energy from the scrubbed gas which is at an elevated pressure, a bypass being provided across this expansion turbine, the turbine unit includes valves for controlling selectively the passage of the gas through the turbine and the bypass.
(c) A control system for the gas pressure at the outlet of the pressurized blast furnace in which the controlled-gap washer constitutes the control element, e.g. with the position of the gap-controlling body of this washer determining the size of the gap and hence the back pressure at the outlet of the blast furnace.
The controlled-gap washing unit can be constituted as a plurality of controlled-gap washers some or each of which may be constituted as an annular-gap washer with a venturi construction, an annular-gap washer in the form of a diffusor washer, or a venturi washers with a corresponding adjustablity. The control of the annular gap width in the annular-gap washer by, for example, the movement of a tapered body in one or another direction, within the venturi constriction, enables control of the pressure differential across the gap and hence of the washing process.
With earlier gas-cleaning apparatus using the controlled-gap washer units described previously, the components of the annular-gap washer are functionally differentiated. A respective controllable-gap washer is provided for each of the two different functions to be performed in the control sense.
The controlled-gap washers are oriented, with respect to the gas flow, either in parallel or in series.
In a parallel connection, a first flow path is provided with a controlled-gap washer connected through a drop separator or the like to the waste-gas duct. In the other flow passage, with an independently functioning controlled-gap washer, there is provided the expansion turbine (see German published application--Auslegeschrift--No. 2,355,457). In this case, the controlled-gap washer of the first path is provided as the exclusive control element for the back pressure at the outlets of the blast furnace, i.e. the control element which exclusively maintains the pressure at the head of the blast furnace constant.
The controlled-gap washer in the other flow path is designed to bring the gas to the desired level of cleanliness for effective use of the expansion turbine, i.e. to minimize deterioration of the turbine. The latter controlled-gap washer, therefore, has a variable-cross-section gap serving only to control the differential washing pressure and hence the washing process.
Where the two functionally distinct controlled-gap washers are connected in tandem or in series, a flow path running to the waste-gas duct is provided and is connected to the last controlled-gap washer while a bypass passage branches from behind the first controlled-gap washer and receives the expansion turbine. Such a system has been described in German published application--Auslegeschrift--No. 2,439,758.
The gas flow through the first controlled-gap washer traverses the expansion turbine after having been scrubbed to the desired degree. The second controlled-gap washer serves at the control element of the circuit for maintaining the gas pressure at the outlet of the pressurized blast furnace constant.
Considerable work with such systems has shown that functionally differentiating the several controlled-gap washers is disadvantageous for many reasons. For example, a plurality of controlled-gap washers must be provided, even if they are only to work alternatively. Surprisingly, even with the differentiation there is always a compromise between the cleaning efficiency and energy recovery of the system in order that that optimum energy recovery and optimum gas cleaning can be obtained simultaneously. In addition, the number of systems which are prone to break down is relatively high and maintenance costs and downtime of the system may be considerable.
To elucidate on this point, it should be noted that a waste-gas cleaning installation of the aforedescribed type has a characteristic gas pressure/gas volume or throughput diagram which can represent the various characteristics of the system. This gas pressure/volume diagram can give, for the expansion turbine, a turbine-insertion characteristic line, a turbine-output (work output) characteristic, a blast furnace characteristic curve and a gas-cleaning characteristic curve which runs parallel to the blast-furnace curve with a distance between the curves determined by the differential pressure of the gas-cleaning operation. The latter value should not fall below a predetermined limit.
The gas-cleaning characteristic curve thus is a transposed replica of the blast-furnace characteristic curve and can also be designated as the "corrected blast-furnace characteristic."
In general and for convenience, in such a diagram, the gas pressure is plotted along the ordinate while the gas volume per hour (throughput) emerging from the pressurized blast furnace is plotted along the abscissa.
All the aforementioned characteristic curves rise from the left-hand corner of the diagram and have their positions on the diagram and relative to one another determined by the operating parameters of the gas-cleaning apparatus and the structure thereof.
Regardless of the particular curve orientations and shapes, the following statements can be made:
(a) The diagram (see FIG. 3) has a region A which lies between the turbine-insertion characteristic curve and the turbine-work output (operating) characteristic curve.
(b) The corrected blast-furnace characteristic curve (gas-cleaning characteristic curve) defines below the turbine-work characteristic curve a region B.
(c) The operating state of the apparatus, when the expansion turbine is operable, can lie a region A or region B and can pass from one region to the other.
(d) The region C to the left of the turbine-insertion characteristic curve will be a region in which the turbine cannot be effectively operated with energy recovery.
(e) Below the region B, moreover, there is a region D in which the expansion turbine will not generate energy and/or in which pressure control is not possible.
The turbine-insertion curve represents pressure/flow-rate values below which the turbine cannot be operated efficiently for energy recovery. The turbine-output characteristic curve represents the pressure/flow-rate values which limit the capacity of the turbine.
In both regions C and D, the expansion turbine must be shut down and the gas-cleaning system operated only to bring about the desired degree of gas cleaning and pressure control at the discharge port of the pressurized blast furnace.
The expansion turbine can be connected to a generator and, in this case, should be driven at a constant speed (as determined by an rpm or angular-velocity controller) if the generator is of the alternating current type and is to have a constant-frequency output. In this case, the expansion turbine can be operated in the region A under a condition in which the volume demand of the turbine exceeds the supply from the pressurized blast furnace. Conversely, the region B of the diagram characterizes the conditions whereby, for a predetermined pressure, the total gas quantity generated by the pressurized blast furnace can no longer be consumed by the turbine.
Because of these relationships, it has, as a practical matter, been difficult if not impossible to provide a satisfactory control of a gas-cleaning installation of the aforedescribed type when the latter is provided with an expansion turbine.